Greenhouse Cooling Concepts Why Cool Greenhouses

Why Cool Greenhouses? • Most places have a summer climate that requires greenhouse cooling – even Vermont • A greenhouse must be capable of cooling in the winter and summer • With passive cooling greenhouses can reach temperatures of 20°F greater (or more) than the air temperature

Passive Cooling • Percent of roof space ventilated has increased over time with design improvements • Success is very crop specific • Even fully retractable roof designs are now available • Cheaper to operate than active cooling systems, but construction costs aren’t less expensive

Effectiveness of Summer Cooling • Fan and pad cooling can lower to 80% of the difference between the wet and dry bulb temperatures • Fog cooling can lower the temperature by nearly all of the difference • Both of these systems are most effective at low humidity

Evaporative Cooling • Works well in most climates; where might it not function effectively? • Based on heat absorption during the evaporation of water • Relatively inexpensive compared to other types of cooling

Pad and Fan Cooling • Available for almost 50 years • Most common system for summer cooling • Originally the pad was composed of wood shreds • Today it is composed of cellulose • Exhaust fans are placed on the opposite wall

Active Summer Cooling System Calculations (Basics) • Fan-and-Pad system • Rate at which warm air must be removed from the greenhouse – Types of pads used – Fan placement – Path of the airstream

Pad Types and Specifications • Excelsior pads (wood fiber) had to be framed in wire mesh for support; required annual replacement • Cross-fluted cellulose is the most popular today, can last up to 10 years – Should be kept from heavy rains – Only move if dry • Other types of pads include aluminum fiber, glass fiber, and plastic fiber • Why are pads thick? and why do they have a cross fluted design?

Cross-fluted cellulose pads • Come in height increments of ft • Available in 2, 4, 6, and 12 inches thick • A 4 -inch-thick pad will handle an air intake of 250 cfm/ft 2; a six inch 350 cfm/ft 2 • By way of comparison excelsior pads can only support an airflow rate of 150 cfm/ft 2 • You want vents over the exterior of the pads to seal the external air source off when active cooling isn’t needed

More Details • Water must be delivered to a 4 -inch pad at the rate of 0. 5 gpm per linear foot of pad • For a 6 -inch thick cellulose pad a 0. 75 gpm per linear foot is required • Longest recommended delivery pipe is 60 ft for the 4 inch system and 50 ft for the 6 inch system 1/8 inch holes every three inches are required for both systems • Holes point upward and release water into an impingement cover – water drips down onto a distribution pad

Rate of Air Exchange • Measured in cfm (cubic feet per minute) • NGMA uses 8 cfm/ft 2 of floor space as a standard • In warmer climates 1 volume per minute recommended roughly 11 -17 cfm/ft 2 • As elevation increases so must the rate of air removal. Why?

Exhaust Fan Placement Rules • Should not be more than 25 ft apart • If the end of the greenhouse is 60 ft wide you will need at least 3 fans • Fans should be evenly spaced at plant height • Place fans on leeward side of the greenhouse • Rules change with multiple houses • Protect fans from weather and provide screening on both sides to protect workers, visitors, and wildlife • Air movement can cause special problems in larger houses

Fog Cooling • 20 -year-old technology • High pressure water delivery system generates a fog of very fine water particles (<10 microns) • Drops evaporate in the air • Even dispersal of the particles means cooling of throughout the greenhouse http: //www. valproducts. com/Air/Evap. Fog. html

Fog Cooling • Initial cost usually close to that of fan and pad cooling systems (water quality determining factor) • Operating cost less than fan-and-pad cooling • Dispersion of water particles in the greenhouse air where they extract heat from the air as they evaporate. • Rate of cooling increases proportionately as water droplet size decreases. • Systems allow near 100 percent cooling efficiency and wet bulb temperatures can essentially be obtained

Fog Cooling II • Exhaust fans still used • Fog nozzles installed just inside the inlet ventilators • Roughly half the exhaust fan capacity of fanand-pad cooling systems is necessary • High water quality is critical • Can also be used with plant propagation systems • – Disease occurrence much lest than with a mist system. Why?

Fog Cooling Advantages • There is less electrical consumption • Heat rise across the greenhouse is controlled • Cooler average temperatures can be achieved across the greenhouse • System is good substitute for mist systems on propagation benches.

Effectiveness of Winter Cooling • Ventilators ‘used’ to be the only way to winter cool – problems • Convection-tube and HAF eliminate horizontal temperature gradient problems • Both modern systems circulate air in the greenhouse

Active Winter Cooling • Convection tube cooling – Exhaust fan turned on – A louver opens in the gable – A pressurizing fan in the end of the polyethylene tubes turns on – Cool air mixes with greenhouse warm air and galls to the floor cooling the plant growing area • Pressurizing fan must move as much air as the exhaust fan. • 2 cfm required

HAF fans • Similar to convection-tube-system • Requires HAF fans in the place of convection tubes • HAF fans can be used for air circulation when neither heating nor cooling is in operation

Integrating of Heating and Cooling Systems Remember: there are some spring and fall days when you may have to use, summer heating, winter cooling and summer cooling systems all on the same day